New Scientist - 2010 Apr 12
First evidence of a 27 day solar signature in noctilucent cloud occurrence frequencyTHE comings and goings of noctilucent or "night shining" clouds in the extreme upper atmosphere may be linked to the sun's rotation.
Noctilucent clouds appear about 80 kilometres above the Earth in each hemisphere's summer. Their extent and brightness varies over days, weeks and years, but no one knows why.
Now Charles Robert of the University of Bremen, Germany, and colleagues think they have an answer. By measuring changes in the light reflected from the clouds, they found that the clouds appear to wax and wane in prevalence over a 27-day cycle. As the sun takes 27 days to rotate around its axis, the team suggest a link
- Journal of Geophysical Research, 2010 March 30, DOI: 10.1029/2009jd012359
This paper presents evidence of a connection between the 27 day modulation of the solar activity and noctilucent cloud (NLC) occurrence frequency as measured by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) and solar backscatter ultraviolet (SBUV) instruments. Observations show anticorrelations significant at the 90% confidence level between noctilucent cloud occurrence rate anomalies and Lyman-α irradiance variation during several seasons in both hemispheres. A superposed epoch analysis confirms these results and also reveals a clear recurrence pattern in noctilucent clouds occurrence anomalies with a ∼27 day period. The superposed epoch analysis also shows that the maximum NLC response in the Northern Hemisphere is clearly localized at 0 day phase lag, while in the Southern Hemisphere the maximum response is broader and occurs at 0 ± 2 day phase lag. Microwave Limb Sounder mesospheric products suggest that the more likely driver for the variation in NLC occurrence is temperature instead of water vapor, but the mechanisms responsible for the observed variations are not yet fully understood.